1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, various types of inkjet printers or laser printers have been provided as image forming apparatuses. For example, an inkjet printer includes one inkjet head for each of black (K), magenta (M), cyan (C) and yellow (Y) to be recorded, i.e., a total of four inkjet heads, each of which can perform linear recording of a sheet width and which are disposed along a moving direction (sub-scanning direction y) of a recording-sheet conveying belt. The inkjet printer forms an image by causing a plurality of units to form toner images of the respective colors and recording the toner images on a sheet of paper on the conveying belt in an overlapping manner.
As the laser printers, for example, a tandem system and a single photoreceptor system are known. A laser printer of the tandem system includes one image forming unit for each of black (K), magenta (M), cyan (C), and yellow (Y) to be recorded, i.e., a total of four image forming units, each of which mainly includes a photoreceptor and a developing unit and which are disposed along a moving direction (hereinafter, the sub-scanning direction y) of a recording-sheet conveying belt or an intermediate transfer belt. The laser printer of the tandem system forms an image by causing the image forming units to form toner images of the respective colors on the photoreceptors and transfer the toner images onto a sheet of paper or the intermediate transfer belt in a superimposed manner. A laser printer of the single photoreceptor system forms an image by repeating, as many times as the number of colors to be used, a process of forming a toner image of one color on a single photoreceptor, transferring the toner image onto a sheet of paper or an intermediate transfer medium, forming another toner image of a different color on the photoreceptor, and transferring the toner image onto the previously-transferred toner image in a superimposed manner.
However, in the image forming apparatuses of both systems, toner images of different colors or different inks are recorded on the same sheet of paper in a superimposed or overlapping manner. Therefore, color deviation or color shade variation may easily occur due to relative positional deviation of the images of the different colors.
In a color image forming apparatus of the tandem system, relative positional deviation of superimposed toner images causes color deviation or color shade variation, resulting in reduced image quality. Therefore, conventionally, to adjust positions (registration) of latent images, registration deviation is detected by using an image recorded on a transfer belt and write timing at positions in the main-/sub-scanning directions is changed to perform adjustment (registration correction).
For example, Japanese Patent Laid-open Publication No. 2003-280317 discloses a positional deviation correction method for correcting deviation of transfer positions. In this method, a pattern is formed as a patch, in which a reference pattern containing a plurality of lines that are formed with black toner serving as a reference color and that are arranged at a predetermined pitch are superimposed on a correction-target color pattern containing a plurality of lines that are formed with color toner and that are arranged at the same pitch as that of the reference patterns. A patch group is formed by sequentially forming a plurality of the patches in a read direction of a detection sensor such that relative positions of the lines are shifted by an arbitrary amount in a pitch direction of the lines. In this method, it is assumed that the black toner serving as the reference color is lastly superimposed onto a transfer medium, and a pattern in which the correction-target color patterns are completely superimposed on or separate from the reference patterns is used as a reference patch. Based on this assumption, patch groups are formed by sequentially arranging a plurality of the patches on the front and rear sides of the reference patch in the read direction of the detection sensor. An intersection of two straight lines is calculated, where the straight lines indicate outputs on both sides of an inflection point of optically-detected values of the patches according to arbitrary shift amounts of the patch groups that are sequentially arranged in a correction pattern in a correction-pattern forming direction. Subsequently, an amount of deviation of the transfer positions on the transfer belt at which toner is transferred from photosensitive drums is calculated based on the intersection, and exposing timing of each of the photosensitive drums is corrected based on the amount of deviation.
If the positional deviation correction method disclosed in Japanese Patent Laid-open Publication No. 2003-280317 is performed before start of printing, it is possible to obtain an image with less positional deviation.
If continuous printing is performed, the temperature of the whole image forming apparatus increases and thermal expansion of units of the image forming apparatus occurs; therefore, positional deviation gradually occurs with respect to a value that has been corrected before printing, resulting in color deviation of an image. However, in the method disclosed in Japanese Patent Laid-open Publication No. 2003-280317, printing is suspended to form patches according to a change in the temperature or according to the amount of printing, a correction amount of positional deviation is calculated by reading an interval between the patches, and the positional deviation of colors in the main-/sub-scanning directions are corrected again.
In Japanese Patent Laid-open Publication No. 2003-280317, it is disclosed that the relationship among the length of each of the patches, the interval between the patches, and a spot diameter formed on the transfer medium by a detection sensor is “(the length of each of the patches)+(the interval between the patches)>(twice the size of the spot diameter on the transfer medium)”. However, in actuality, a light emission pattern of spot light from the detection sensor (a light emitting diode (LED)) is, in some cases, non-uniform and horizontally asymmetry.
This will be explained below with reference to FIG. 14. Illustrated in (a) of FIG. 14 is a schematic diagram of a part of the patch disclosed in Japanese Patent Laid-open Publication No. 2003-280317. Illustrated in (b) of FIG. 14 is a graph (the horizontal axis represents a distance) of a light emission pattern of light applied to the transfer belt. Illustrated in (c) of FIG. 14 is a graph in which peak values of the detected diffused output voltages are plotted.
When a horizontally-symmetric light emission pattern indicated by a bold line A in (b) of FIG. 14 is obtained, a color toner portion is shifted in a region where the amount of light is stable. Therefore, the detection sensor can stably receive diffused light from the patches without a variation in the amount of diffused reflected light.
However, as indicated by a dashed line B in (b) of FIG. 14, in actuality, the light emission pattern of the detection sensor may be horizontally deformed (asymmetry) due to an attachment error of a light source or an attachment error of an optical system. For example, when a light emission pattern indicated by the dashed line B is obtained, and if color toner is gradually shifted to the left on a patch-by-patch basis, the position of receiving light from the detection sensor is gradually shifted to the left and the amount of light received from the detection sensor is reduced.
As illustrated in (c) of FIG. 14, in the case of the dashed line B, the amount of light applied from the detection sensor for the first patch (the leftmost plot in (c) of FIG. 14) is obtained at the flat position. However, because the amount of light on the left side is reduced as the color patch is shifted to the left, the diffused light is gradually reduced relative to the bold line A.
However, in Japanese Patent Laid-open Publication No. 2003-280317, the value of the detection sensor is detected on the assumption that the amount of applied light does not change, and the amount of positional deviation is calculated based on the detected value. Therefore, for example, in the case of the dashed line B, the amount of applied light changes along with the shift of the color toner, and the intersection of two straight lines indicating outputs on the both sides of the inflection point described above becomes an intersection indicated on the dashed line B in (c) of FIG. 14 resulting in an error. Therefore, in some cases, a result different from actual positional deviation is calculated. If a wrong correction amount is calculated, positional deviation cannot accurately be corrected, and in some cases, color deviation gets even worse after correction of the positional deviation.
Therefore, there is a need for an image forming apparatus capable of obtaining a print image of good quality with less color deviation or color shade variation.